Electron beam microcolumns based on microfabricated electron optical components and field emission sources operating under the scanning tunneling microscope (STM) aided alignment principle were first introduced in the late 1980s. Electron beam microcolumns are used to form a finely focused electron beam. See Chang, T. et al., "Electron-Beam Microcolumns for Lithography and Related Applications" J. Vac. Sci. Technology, B 14(6), pp. 3774-3781, November/December 1996, and Lee, K. et al, "High Aspect Ratio Aligned Multilayer Microstructure Fabrication" J. Vac. Sci. Technology, B 12(6), pp. 3425-3430, November/December 1994, incorporated by reference herein. These columns offer the advantages of extremely high resolution with improved beam current, small physical size, and low cost, and can be used in a wide variety of applications, such as electron beam lithography.
Microcolumns are high-aspect-ratio micromechanical structures comprised of microlenses and deflectors. The microlenses are multilayers of silicon chips (with membrane windows for the lens electrodes) or silicon membranes spaced apart by thick, 100-500 .mu.m, insulating layers. The lenses have bore diameters that vary from a few to several hundred micrometers. For optimum performance, the roundness and edge acuity of the bores are required to be in the nanometer regime and alignment accuracy between components on the order of less than 1 .mu.m.
Electrodes of the microlenses can be made from 1 to 2.5 .mu.m thick silicon membranes by electron-beam lithography and reactive-ion etching (RIE). The starting material is a 4 inch diameter and 500-.mu.m-thick double-sided polished wafer containing arrays of 7 mm.times.7 mm chips. At the center of each chip is a 1 mm.times.1 mm membrane formed by wet isotropic etching using in preferred form either a highly boron doped or a reverse-biased p/n junction etch stop.
Assembly of the lenses and the column typically involves stacking together silicon components and Pyrex spacers and using anodic bonding.
FIG. 1 shows a cross-sectional view of a 1 kV microcolumn based on the well-known STM aligned field emission (SAFE) concept, showing source section 1 and Einzel lens section 3. Scanning tunneling microscope (STM) scanner 5 emits an electron beam 6 in the direction of sample plane 25. The beam 6 first passes through the source 1, composed of silicon microlenses, 5 .mu.m diameter extractor 7, 100 .mu.m diameter anode 11, and 2.5 .mu.m diameter limiting aperture 13. The three microlenses are separated by two insulating spacers 9. The insulating spacers 9 are preferably formed of Pyrex, but could be made of any other suitable insulator, such as SD-2 glass made by Hoya. The source 1 is mounted on aluminum mounting base 15, which contains an octupole scanner/stigmator 17. The electron beam 6 then passes through the Einzel lens 3, which is composed of two 100-200 .mu.m diameter silicon microlenses 19 and 23 with a 1-1.5 .mu.m thick free-standing silicon membrane 21 disposed therebetween. Each silicon layer is again separated by insulating spacers 9. The electron beam 6 then passes on to sample plane 25 and channeltron detector 27.
The source 1 and Einzel lens 3 are shown expanded and in greater detail in FIGS. 2(a)-(b) with similar reference numbers identifying the same structures.
The conventional approach to bonding the insulating and microlens layers of the microcolumn involves the use of anodic bonding. Anodic bonding is an electrochemical process for heat sealing of glass to metal and semiconductors, as shown in FIGS. 3(a) and (b). At elevated temperatures (300-600.degree. C.), Na.sub.2 O in the Pyrex or other glass dissociates to form sodium and oxygen ions. By applying a potential with voltage source 52 between a first silicon layer 53 and a glass insulation layer 55, sodium ions in the glass migrate from the interface in a direction indicated by arrow 63, while uncompensated oxygen anions 61 move toward the induced positive charge 59 of the silicon anode to form chemical bonds.
This process, previously used for single sided bonding only, has been extended to multilayer bonding. After the first silicon-to-glass bond, another silicon chip or membrane can be bonded to the free surface of the glass by reversing the applied potential, as shown in FIG. 3(b). In this case, second silicon layer 57 is placed atop glass insulation layer 55 and an opposite potential is applied by voltage source 52. Here, the induced positive charge 59 causes the sodium ions to migrate downward in the direction of arrow 63, causing the oxygen anions 61 to form chemical bonds with the second silicon layer 57. To achieve satisfactory multilayer bonding, special attention has to be given to the control of temperature, the applied voltage, the bonding time, and, in particular, the surface condition of the layers.
One disadvantage of the anodic bonding process is that it must be conducted at elevated temperatures, which typically requires several hours of heat-up (to approximately 400.degree. C.) and cool-down time, as well as a physical connection of a high voltage probe, during which time drift, bond-induced shift, and expansion can cause the alignment to degrade. This process must then be repeated for each additional layer.
Accordingly, it is clear that there is a need for a method of forming microcolumn structures that avoids the burden of anodically bonding each layer of glass to silicon.